The present invention relates to a combustion method and a combustor for gas turbine and, more particularly, to a combustion method and a combustor for reducing emission of nitrogen oxides (referred to as NOx, hereinafter) when LNG (Liquefied Natural Gas) or the similar gaseous fuel such as coal gas is used.
The NOx and carbon monoxide (referred to as CO, hereinafter) contained by the exhaust gas emitted from a gas turbine are inherently noxious, and constitute one of the causes of air pollution and photochemical smog. Recently, there is an increasing demand for NOx reduction to prevent destruction of natural environment. This demand is severe particularly from the social point of view. In order to fully meet the regulation, it is necessary to reduce the level of NOx down to about 1/10 or less than the present level.
In general, it is said that the NOx is generated in a high flame temperature region in a combustor and that the generation of NOx can effectively be reduced by lowering the flame temperature.
It has been proposed that a gas turbine, into which excess air is supplied; is adapted to make a combustion at a low temperature, thanks to the supply of a part of the excess air to the high flame temperature region. This type of combustion method is usually referred to as low-temperature lean-mixture combustion. This conventional low-temperature lean-mixture combustion, however, cannot provide remarkable reduction of NOx emission to such a level as to meet the present strict regulation, for the reasons described hereinafter.
In the low-temperature and lean-mixture combustion, although excess air is supplied, there must be region where optimum combustion is carried out to give a high flame temperature, consequently, NOx is inevitably generated in such high flame temperature region.
Increased supply of the excess air for the reduction of NOx will produce an excessively cooled area, so that CO and other unburnt substance will be increased undesirably, although a further reduction of NOx may be achieved. In the worst case, the supply of excessive air may cause a discontinuation of flame or misfire.
U.S. Pat. No. 4,058,977 proposes a low NOx emission combustor of swirl type chambers by subjecting the air passing through the engine to NOx producing elevated temperatures for minimal periods of time and by establishing a controlled ignition lag so as to permit molecular premixing between the so-called vitiated swirling, prevaporized fuel-air mixture and swirling combustion air to establish controlled autoignition so as to produce high-rate, lean burning in the primary combustion chamber, where the term "vitiated" is used in describing a fuel and air mixture, where the oxygen available for combustion in the air or mixture is less than the normal 21%, that is, a mixture of reduced oxygen content. The aforementioned patent relates to a pilot burner structure based on the combustion of vitiated mixture of vaporized liquid fuel and air to use the combustion product from the burner for flash vaporization of fuel droplets so as to effect ignition lag until supplemental swirling combustion air can be introduced to increase the oxygen content. That is, the Patent is directed to low NOx emission from liquid fuel, and not directed to the use of gaseous fuel.